Many of the active substances of medicines, i.e., pharmaceutical active ingredients, have poor water solubility. Such substances are poorly absorbed from the alimentary tract, and the bioavailability and drug efficacy expression are easily reduced or are subject to fluctuation. For this reason, in preclinical tests that evaluate drug efficacy or obtain biopharmaceutical parameters using lab animals or the like, the pharmaceutical active ingredients are often dissolved in some solvent to make them more easily absorbed. For a poorly soluble pharmaceutical active ingredient, it is possible to use a polyethylene glycol having relatively low molecular weight and a derivative thereof, a polyoxyethylene sorbitan fatty acid ester, a fatty acid having 6 to 12 carbon atoms or a salt thereof, polyoxyethylene castor oil, a diethylene glycol derivative, or the like. However, these solvents are usually in liquid form and not easily processed into tablets. Therefore, it is necessary to consider the ultimate dosage form of these solvents for sale in the market. If these solvents could be directly formulated into pharmaceutical preparations, the time required for formulation could be greatly shortened. A capsule is highly anticipated to serve as such a dosage form.
Capsules hitherto known are those produced using gelatin or a cellulose derivative as a base material. When a known gelatin hard capsule is filled with a polyethylene glycol having a weight average molecular weight of 400 (PEG 400), the moisture in the capsule film migrates into the solvent, causing the capsule to break (see Non-Patent Literature (NPL) 1). Moreover, in known cellulose derivative-based capsules, the aforementioned solvents act as plasticizers, causing them to, for example, permeate the capsule film and be exuded to the capsule surface.
In order to solve such problems, Patent Literature (PTL) 1 discloses a hard capsule comprising a film that comprises a specific polyvinyl alcohol copolymer and polyvinyl alcohol. This hard capsule has improved fracture resistance and impact resistance. However, because of the low solubility of polyvinyl alcohol particularly under a low temperature, it takes a long time to dissolve this hard capsule. Therefore, the capsule was not suitable to contain, in particular, medicaments that must be quickly absorbed.